Method for controlling transmission power in cellular system

ABSTRACT

A method for controlling transmission power of a base station in a cellular system includes: receiving a reference signal from at least one neighbor base station that is located around the base station; estimating a radio environment between the base station and the neighbor base station by using the receiving power of the reference signal; and controlling cell coverage of the base station by determining the transmission power based on the radio environment.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2009-0124876 filed in the Korean IntellectualProperty Office on Dec. 15, 2009, the entire contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a method for controlling transmissionpower in a cellular system.

(b) Description of the Related Art

A cellular system has a cell structure to efficiently configure asystem. FIG. 1 is an exemplified diagram showing a cellular system.

Referring to FIG. 1, in a cellular system, a cell 1 is surrounded by aplurality of cells (Cell 2, Cell 3, Cell 4, Cell 5, Cell 6, and Cell 7).One cell includes a base station (BS) and at least one set of userequipment (UE). In a downlink, a transmitter may be a part of the basestation and a receiver may be a part of the user equipment. In anuplink, the transmitter may be a part of the user equipment and thereceiver may be a part of the base station.

The cell is an area where one base station provides communicationservices. A multi-cell may be formed by disposing the base stationhaving at least one cell in plural. The base station that provides thecommunication services to the user equipment may be referred to aserving base station (Serving BS), and a base station located around theserving base station may be referred to a neighbor base station(Neighbor BS). The cell of the serving base station is referred to as aserving cell, and the cell of the neighbor base station is referred toas a neighbor cell.

In a general cellular system, a plurality of base stations are uniformlydisposed and each cell coverage is divided in a hexagonal comb shape.Offline work to dispose the plurality of base stations is performed inconsideration of the maximum transmission power of the base station, theuser density, the topography, and the maximum capacity of the basestation. Any user equipment measures receiving power from the pluralityof base stations by using a reference signal, and is connected to thebase station having the strongest receiving power. The reference signalis a primary common pilot channel (P-CPICH) signal in, for example, a3^(rd) generation partnership project (3GPP) system or a preamble signalin a worldwide interoperability for microwave access (WIMAX) system.Each base station transmits the reference signal, including anindicator. The user equipment detects the reference signal to find thebase station, and compares the receiving power from the plurality ofbase stations to select the base station to be connected.

Meanwhile, unlike the general cellular system, a field requiring asystem operation that can non-uniformly dispose a plurality of basestations or change the position of the base station has emerged. Anexample thereof may include a base station (hereinafter referred to as amilitary base station) in a military tactics communication system. Onemilitary base station may be disposed for each troop. Each troop isdisposed in a tactical area, and the military base station in charge ofeach troop may be located in the campsite of the troop. In addition, theposition of the military base station may be changed according to themovement of the troop. Therefore, it may be difficult for the militarybase station to have a uniform disposition like the general cellularsystem. A wireless mesh network is formed between the military basestations. In addition, the military base station generally includes aglobal positioning system (GPS) using a satellite to obtain position andtime information according to the characteristics of the military systemthat requires precision measurements.

FIG. 2 is an exemplified diagram showing a cellular system in which aplurality of base stations are non-uniformly disposed.

Referring to FIG. 2, the position of the base station is represented bya dot and a cell coverage is represented by a circle when each basestation transmits at the same transmission power. Many areas wherecoverage is overlapped between the neighbor base stations are generateddue to the non-uniform disposition of the base station. In addition,many outage areas are generated between cells. Therefore, the inter-cellinterference and path loss according to a distance between the basestation and the user equipment may be large. The data rate and thetransmission speed can be deteriorate due to the inter-cell interferenceand the path loss.

Therefore, in the cellular system where the plurality of base stationscan be non-uniformly disposed with respect to each other or the positionof the base station can be changed, a need exists for a technology thatcan variably operate the cell coverage.

The above information disclosed in this Background period is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE INVENTION

The present invention has been made in an effort to provide a technologyof variably operating a cell coverage in a cellular system where aplurality of base stations can be non-uniformly disposed with respect toeach other or the position of the base station can be changed.

An exemplary embodiment of the present invention provides a method forcontrolling transmission power of a base station in a cellular system,including: receiving a reference signal from at least one neighbor basestation that is located around the base station; estimating a radioenvironment between the base station and the neighbor base station byusing the receiving power of the reference signal; and controlling acell coverage of the base station by determining the transmission powerbased on the radio environment.

Another exemplary embodiment of the present invention provides a methodfor controlling transmission power of a plurality of base station in acontrol station of a cellular system, including: receiving a position ofeach base station from the plurality of base stations; determiningtransmission power of each base station in consideration of the positionof each base station in order to control each cell coverage; andinforming each base station of the control information on thetransmission power.

Yet another exemplary embodiment of the present invention provides: areference signal receiver that receives a reference signal from aneighbor base station; a transmission power calculator that calculatestransmission power by using the received reference signal; and a powercontroller that transmits the reference signal to the neighbor basestation at a predetermined power and transmits a signal to an userequipment in a cell based on the transmission power calculated in thetransmission power calculator.

According to the exemplary embodiment of the present invention, the cellcoverage in the cellular system where the plurality of base stations canbe non-uniformly disposed with respect to each other or the position ofthe base station can be changed can be variably operated, thereby makingit possible to reduce the inter-cell interference and the path loss. Inaddition, the deterioration of the data rate and the transmission speeddue to the inter-cell interference and the path loss can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplified diagram showing a cellular system;

FIG. 2 is an exemplified diagram showing a cellular system in which aplurality of base stations are non-uniformly disposed;

FIG. 3 is a diagram showing a cellular system that controls cellcoverage according to an exemplary embodiment of the present invention;

FIG. 4 is a diagram showing a cellular system according to an exemplaryembodiment of the present invention;

FIG. 5 is a flowchart showing a method for controlling transmissionpower by the central control scheme according to the exemplaryembodiment of the present invention;

FIG. 6 is a schematic block diagram of a base station according toanother exemplary embodiment of the present invention;

FIG. 7 is a flowchart showing a method for controlling transmissionpower by a distributed control scheme according to an exemplaryembodiment of the present invention;

FIG. 8 is a flowchart showing a method for calculating transmissionpower using a reference signal received by a neighbor base stationaccording to an exemplary embodiment of the present invention;

FIG. 9 is a graph showing a path loss according to a distance;

FIG. 10 is a diagram showing a period for transmitting the referencesignal according to an exemplary embodiment of the present invention;and

FIG. 11 is a diagram showing a frame structure in which a period fortransmitting the reference signal according to an exemplary embodimentof the present invention is inserted.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following detailed description, only certain exemplaryembodiments of the present invention have been shown and described,simply by way of illustration. As those skilled in the art wouldrealize, the described embodiments may be modified in various differentways, all without departing from the spirit or scope of the presentinvention. Accordingly, the drawings and description are to be regardedas illustrative in nature and not restrictive. Like reference numeralsdesignate like elements throughout the specification.

In the specification, unless explicitly described to the contrary, theword “comprise” and variations such as “comprises” or “comprising” willbe understood to imply the inclusion of stated elements but not theexclusion of any other elements.

In the specification, a terminal can be fixed or moved, and maydesignate a mobile station (MS), a mobile terminal (MT), a subscriberstation (SS), a portable subscriber station (PSS), user equipment (UE),an access terminal (AT), etc., and may include the entire or partialfunctions of the mobile station, the mobile terminal, the subscriberstation, the portable subscriber station, the user equipment, the accessterminal, etc.

A base station (BS) may designate an access point (AP), a radio accessstation (RAS), a node B, an evolved node-B (eNodeB), atransmitting/receiving base station (BTS), a mobile multihop relay(MMR)-BS, etc., and may include the entire or partial functions of theaccess point, the radio access station, the nodeB, the eNodeB, thetransmitting/receiving base station, the MMR-BS, etc.

FIG. 3 is a diagram showing a cellular system that controls cellcoverage according to an exemplary embodiment of the present invention.

Referring to FIG. 3, a plurality of base stations, which are representedby a dot, are non-uniformly disposed with respect to each other, and theoverlapping area and the outage area of the cell coverage is minimized.For this purpose, the transmission power control can be considered. Wheneach base station uses different power transmission to perform downlinktransmission, cell coverage of different sizes may be formed. Therefore,when each base station uses transmission power of an appropriatemagnitude to perform the downlink transmission, the overlapped area andthe outage area of the cell coverage can be minimized.

Hereinafter, a method for controlling transmission power to control thecell coverage in the base station will be described.

FIG. 4 is a diagram showing a cellular system according to an exemplaryembodiment of the present invention.

Referring to FIG. 4, the cellular system includes plurality of basestations 100-1, 100-2, and 100-3, a control station 300, and a satellite200 that connects the plurality of base stations 100-1, 100-2, and 100-3to the control station 300 by satellite communication.

The position of each of the base stations 100-1, 100-2, and 100-3 ismeasured and the measured position is transmitted to the satellite 200.The satellite 200 informs the control station 300 of the position of thebase stations. The control station 300 determines the transmission powerof the base stations 100-1, 100-2, and 100-3 in consideration of theposition of the base station transmitted from the satellite 200. Thecontrol station 300 transmits the control information on the determinedtransmission power to each base station 100-1, 100-2, and 100-3. Thecontrol information on the transmission power may include the magnitudeof the transmission power. Each base station 100-1, 100-2, and 100-3performs the downlink transmission to the user equipment based on thetransmission power control information received from the control station300.

Although FIG. 4 shows a cellular system using the satellitecommunication 200, a cellular system of other schemes can be used. Forexample, when the control station 300 is connected to the base station100-1, 100-2, and 100-3 by wireless or wire or when the plurality ofbase stations 100-1, 100-2, 100-3 are connected to a mesh network, acentral control scheme using one base station as the control station 300can be used.

FIG. 5 is a flowchart showing a method for controlling transmissionpower by the central control scheme according to the exemplaryembodiment of the present invention.

Referring to FIG. 5, the position of each base station 100-1, 100-2, and100-3 is measured (S100, S101, and S102), and the measured position istransmitted to the control station 300 (S110, S111, and S112).

The control station 300 determines the transmission power of each basestation in consideration of the position of the received base stations100-1, 100-2, and 100-3 (S120). For example, when all the base stations100-1, 100-2, and 100-3 are densely positioned, the control station 300can be set to lower the transmission power of all the base stations100-1, 100-2, and 100-3. As another example, when the base stations100-1, 100-2, and 100-3 are located further away from each other, thecontrol station 300 can be set to increase the transmission power of allthe base stations 100-1, 100-2, and 100-3. As another example, when somebase stations 100-1 and 100-2 are densely located and a remaining basestation 100-3 is located further away from the base stations 100-1 and100-2, the control station 300 relatively lowers the transmission powerof base stations 100-1 and 100-2 and relatively increases thetransmission power of the remaining base station 100-3.

The control station 300 can consider the position of each of the basestations 100-1, 100-2, and 100-3 as well as the channel state in orderto determine the transmission power of each base station 100-1, 100-2,and 100-3 at step S120. The channel state may be represented by at leastone of information such as a signal to noise ratio (SNR), a signal tointerference and noise ratio (SINR), a channel quality indicator (CQI),a quality of service (QoS), etc.

The control station 300 transmits the control information on thetransmission power of each base station 100-1, 100-2, and 100-3 to eachof the base stations 100-1, 100-2, and 100-3 (S130, S131, and S132).Each base station 100-1, 100-2, and 100-3 sets its own transmissionpower based on the transmission power control information received fromthe control station 300 (S140, S141, and S142).

The transmission power control process of FIG. 5 can be periodically oraperiodically repeated according to a position change of the basestation. Further, the transmission power control may be persistent,semi-persistent, or event-triggered.

As described above, according to the power control method in the centralcontrol scheme, the base station determines the transmission power(i.e., cell coverage) based on the transmission power controlinformation that is provided from the control station such that when thebase station is newly installed and starts operation or the base stationis moved at a low speed, the overlap of the cell coverage and theoccurrence of the outage can be minimized.

The transmission power cannot be controlled by the central controlscheme, but each base station can determined the transmission power.This transmission power control scheme is referred to as a distributedcontrol scheme. The case that cannot control the transmission power bythe central control scheme may include, for example, a case where thebase station does not have a location estimating apparatus, a case wherethe base station is moved at a high speed, a case where the base stationdoes not have a satellite communication apparatus, a case where the basestation is not connected by a mesh network, etc. Hereinafter, thedistributed control scheme will be described in detail with reference toFIGS. 6 to 11.

FIG. 6 is a schematic block diagram of a base station according toanother exemplary embodiment of the present invention, and FIG. 7 is aflowchart showing a method for controlling transmission power by adistributed control scheme according to an exemplary embodiment of thepresent invention. FIG. 8 is a flowchart showing a method forcalculating transmission power using a reference signal received by aneighbor base station according to an exemplary embodiment of thepresent invention.

Referring to FIG. 6, a base station 400 includes a reference signalreceiver 410, a transmission power calculator 420, a power controller430, a signal generator 440, and a signal amplifier 450.

Referring to FIGS. 6 and 7, a reference signal receiver 410 receives areference signal (RS) from a neighbor base station (S200). The referencesignal is a signal that is used for estimating the channel, and is asignal that is known by both a transmitting side and a receiving side.The reference signal can be transmitted through a primary common pilotchannel or a preamble. The base station 400 transmits a reference signalto a neighbor base station. The reference signal receiver 410 candirectly receive the reference signal from the neighbor base station orcan receive the reference signal via the base station 400 and thereceiving apparatus that is connected by wire or wirelessly. Thereceiving apparatus that is connected to the base station may be ageneral terminal that is used in the cellular system or a receiving onlyterminal.

The base station 400 and the neighbor base station transmit thereference signal at a predetermined same power. The plurality of basestations can transmit the reference signal through different radioresources. Herein, the radio resource may be at least one of a timeresource, a frequency resource, and a code resource. In addition, anindicator can be allocated to the reference signal to differentiate thebase station that transmits the reference signal, and the indicator maybe generated for each base station cell by using the cell indicator.

Referring back to FIGS. 6 and 7, the transmission power calculator 420estimates the radio environment of the base station 400 by using thereceived reference signal (S210). The radio environment may be theposition of the base stations, the distance between the base stations,the channel state, etc. The channel state may be represented by at leastone of information such as a signal to noise ratio (SNR), a signal tointerference and noise ratio (SINR), a channel quality indicator (CQI),a quality of service (QoS), etc.

The transmission power calculator 420 calculates the transmission powerbased on the estimated radio environment (S220).

The power controller 430 of the base station 400 controls the power ofthe base station based on the calculated transmission power (S230). Thepower controller 430 transmits the reference signal by the predeterminedpower. The power controller 430 allows the base station 400 to transmita general signal other than the reference signal at the transmissionpower calculated at step S220.

A reference signal generator 441 in the signal generator 440 of the basestation 400 generates a reference signal that is used to estimate theradio environment of the neighbor base stations, a general signalgenerator 442 generates a general signal other than the referencesignal, and the signal amplifier 450 amplifies the general signal at thetransmission power that corresponds to the control of the powercontroller 430 (S240). The general signal is a control signal or a datasignal between the base station 400 and the terminal in the cell.

Therefore, the base station can control the transmission power based onthe radio environment in addition to the positional relationship of theneighbor base stations and can efficiently control the cell coverage.The above-mentioned transmission power control process can beperiodically or aperiodically repeated according to a change in positionof the base station. Alternatively, the transmission power control maybe persistent, semi-persistent, or event-triggered.

Referring to FIG. 8, the transmission power calculator 420 of the basestation 400 obtains a correlation value between the reference signaltransmitted from the neighbor base station and the reference signalreceived from the neighbor base station (S300).

The transmission power calculator 420 of the base station 400 comparesthe correlation value with the predetermined threshold value (S310). Thethreshold value may be cell common or cell specific, and can be used todetermine whether there is an active base station that has an effect onthe base station 400 above a predetermined level. For example, the basestation can determine that there is the active base station if thecorrelation value is larger than the threshold value and that there isno active base station if the correlation value is smaller than thethreshold value.

If it is determined that there is an active base station, thetransmission power calculator 420 of the base station 400 stores thereceiving power of the reference signal received from the active basestation in the receiving power table (S320). Step S300 to step S320 mayprogress for each neighbor base station. If it is determined that thereis no active base station, the base station can be operated as anindependent cell.

The transmission power calculator 420 of the base station 400 determinesthe transmission power of the reference signal that is stored in thetable (S330).

Generally, Equation 1 is established between the transmission power andthe receiving power.

P _(t)[dB]=P _(r) −PL+AG _(t) +AG _(r)  (Equation 1)

Herein, P_(t) is transmission power, P_(r) is receiving power, PL ispath loss, AG_(t) is transmitting antenna gain, and AG_(r) is receivingantenna gain. The path loss may be represented by a function of a heightof the transmitting antenna and the receiving antenna, a distancebetween the transmitting base station and the receiving base station,and a frequency.

According to Equation 1, the transmission power calculator 420calculates the path loss (PL) between the base stations to determine thetransmission power (S331).

Generally, the path loss between the base station and the terminal in acity may be represented by the following Equation 2 according to a costmodel.

$\begin{matrix}{{{PL}\left\lceil {dB} \right\rceil} = {{\left( {44.9 - {6.55{\log_{10}\left( h_{bg} \right)}}} \right){\log_{10}\left( \frac{d}{1000} \right)}} + 45.5 + {\left( {35.46 - {1.1h_{ms}}} \right){\log_{10}\left( f_{c} \right)}} - {13.82{\log_{10}\left( h_{bs} \right)}} + {0.7h_{ms}} + C}} & \left( {{Equation}\mspace{14mu} 2} \right)\end{matrix}$

Herein, h_(bs) is a height of the base station, h_(ms) is a height of aterminal, d is a distance between the base station and the terminal, andf is a central frequency. C, which is a constant, is 3. For example, ifh_(bs)=32 m, h_(ms)=1.5 m, and f=1900 MHz, Equation 2 may be representedby a function of a distance according to the following Equation 3.

PL(d)[dB]=31.5+35 log₁₀(d)  (Equation 3)

Equations 2 and 3 are applied between the terminal and the base station,but Equations 2 and 3 are modified into a form of using the neighborbase station instead of the terminal, such that they can be appliedbetween the base stations.

The path loss according to the distance of Equation 3 can be representedby a graph of FIG. 9. In FIG. 9, the horizontal axis represents adistance and the vertical axis represents a path loss. Most path losseshave a form similar to the graph of FIG. 9. The equation representingthe path loss may be differently represented according to the radioenvironment of the area in which the base station is installed.Therefore, the equation representing the path loss can be changedaccording to the position of the base station. Equation 3 can beappropriately changed according to accumulated experimental results andexperience.

The following Equation 4 is another equation that represents the pathloss.

PL(d)=P _(t,r) −P _(r,r) +AG _(t) +AG _(r)  (Equation 4)

Herein, P_(t,r) is the transmission power of the reference signal andP_(r,r) is the receiving power of the reference signal. Since thetransmission power of the reference signal is previously set to be thesame between the plurality of base stations, and the transmittingantenna gain and the receiving antenna gain are already known, the pathloss of the reference signal can be determined from Equation 4.

The transmission power calculator 420 of the base station 400 estimatesthe distance between the base stations by using the measured path loss(S332). The distance between the base stations can be estimated bysubstituting it into the graph of FIG. 9.

The transmission power calculator 420 of the base station 400 controlsthe cell coverage by using the estimated distance between the basestations (S333), and determines the transmission power so that thetransmission power becomes the receiving power having a desiredmagnitude at the cell boundary (S334). The following Equation 5determines the transmission power so that the transmission power becomesthe receiving power having a desired magnitude at a middle point betweentwo base stations.

P _(t,k) =P ^(ce) +PL(d/2)−(AG _(t) −AG _(r))  (Equation 5)

Herein, P_(t,k) is the transmission power of the k-th base station,P_(ce) is the magnitude of the desired receiving power at the cellboundary, and PL(d/2) is the path loss at a middle point between twobase stations. PL (d/2) can be estimated by using the graph of FIG. 9.

When a point nearer any one base station than the middle point betweentwo base stations is considered as the cell boundary, the followingequation can be used.

P _(t,k) =P _(ce) +PL(d _(x))−(AG _(t) +AG _(r))  (Equation 6)

Herein, P_(t,k) is the transmission power of the k-th base station,P_(ce) is the magnitude of the desired receiving power at the cellboundary, and PL(d_(x)) is the path loss at a distance corresponding tox(0<x<1) times the distance between the base stations. PL(d_(x)) can beestimated by using the graph of FIG. 9.

When the plurality of active base stations exist around any basestation, the transmission power for determining the cell coverage can becalculated based on at least one of the plurality of active basestations. For example, an active base station where the receiving powerof the reference signal is minimum may be a reference, or an active basestation where the receiving power of the reference signal is maximum maybe a reference. Alternatively, the average value of the receiving powerof the plurality of reference signals stored in the table may be areference.

Next, a method for transmitting the reference signal will be describedin detail with reference to FIGS. 10 and 11.

FIG. 10 is a diagram showing a period for transmitting the referencesignal according to an exemplary embodiment of the present invention,and FIG. 11 is a diagram showing a frame structure in which a period fortransmitting the reference signal according to an exemplary embodimentof the present invention is inserted.

In order to identify the reference signal transmitted from each basestation, the same resource should not be allocated between base stationswithin a predetermined range. The resource for transmitting thereference signal may be previously allocated to each base station.

Referring to FIG. 10, some time resources P1, P4, and P8 of the entiretime resources P1, P2, P3, P4, P5, P6, P7, P8, and P9 for transmittingthe reference signal are used by three base stations BS1, BS2, and BS3.For example, the time resource P1 is used for transmitting the referencesignal of the base station BS1, the time resource P4 is used fortransmitting the reference signal of the base station BS2, and the timeresource P8 is used for transmitting the reference signal of the basestation BS3. Therefore, a new base station selects some time resourcesof the remaining time resources P2, P3, P5, P6, P7, and P9, therebymaking it possible to transmit the reference signal.

Although FIG. 10 illustrates the period allocation for transmitting thereference signal based on the time resource, it is not limited thereto.A period for transmitting the reference signal can be allocated by usingthe time resource as well as the frequency resource and the coderesource. In addition, a period for transmitting the reference signalcan be allocated by combining at least two of the time resource, thefrequency resource, and the code resource.

Referring to FIG. 11, the cell coverage control frame (CCC frame) isperiodically and aperiodically inserted between the general frames. Thegeneral frame may be one unit of a superframe, a radioframe, a frame, asubframe, and a slot. The general frame is an area where the generalcontrol signal and the data signal between the base station and theterminal are transmitted.

The CCC frame includes a period for transmitting the reference signaland a period for transmitting the cell coverage control message. Theperiod for transmitting the reference signal may be a period shown inFIG. 10, as an example. Each base station transmits the reference signalto the neighbor base station through the period for transmitting thereference signal in the CCC frame, or receives the reference signal fromthe neighbor base station. Some of the periods for transmitting thereference signal are allocated to each base station. Each base stationtransmits and receives a necessary message to and from the cell coveragecontrol through the period for transmitting the cell coverage controlmessage in the CCC frame.

The above-mentioned exemplary embodiments of the present invention arenot embodied only by an apparatus and method. Alternatively, theabove-mentioned exemplary embodiments may be embodied by a programperforming functions that correspond to the configuration of theexemplary embodiments of the present invention, or a recording medium onwhich the program is recorded.

While this invention has been described in connection with what ispresently considered to be practical exemplary embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method for controlling transmission power of a base station in acellular system, comprising: receiving a reference signal from at leastone neighbor base station that is located around the base station;estimating a radio environment between the base station and the neighborbase station by using the receiving power of the reference signal; andcontrolling a cell coverage of the base station by determining thetransmission power based on the radio environment.
 2. The method ofclaim 1, wherein the radio environment includes a distance between thebase station and the neighbor base station.
 3. The method of claim 1,wherein the reference signal is allocated to different radio resourcesfor each base station.
 4. The method of claim 3, wherein the radioresources are at least one of a time resource, a frequency resource, anda code resource.
 5. The method of claim 1, wherein the reference signalincludes an indicator to identify the base station.
 6. The method ofclaim 1, wherein the reference signal transmitted by the base stationand the reference signal transmitted by the neighbor base station aretransmitted at power having the same magnitude.
 7. The method of claim1, wherein the estimating the radio environment comprises: obtaining thepath loss between the base station and the neighbor base station fromthe receiving power of the reference signal; and estimating a distancebetween the base station and the neighbor base station from the pathloss, and the controlling the cell coverage comprises: controlling thecell coverage based on the estimated distance; and determining thetransmission power to obtain receiving power having a desired magnitudeat a boundary of the controlled cell coverage.
 8. The method of claim 7,further comprising obtaining a correlation value between the referencesignal transmitted by the base station and the reference signal receivedfrom the neighbor base station and comparing the correlation value witha predetermined threshold value to determine whether there is a neighborbase station.
 9. A method for controlling transmission power of aplurality of base station in a control station of a cellular system,comprising: receiving a position of each base station from the pluralityof base stations; determining transmission power of each base station inconsideration of the position of each base station in order to controleach cell coverage; and informing each base station of the controlinformation on the transmission power.
 10. The method of claim 9,wherein the control station determines the transmission power by furtherconsidering a channel state.
 11. The method of claim 10, wherein thechannel state corresponds to at least one of a signal to noise ratio(SNR), a signal to interference and noise ratio (SINR), and a channelquality indicator (CQI).
 12. A base station, comprising: a referencesignal receiver that receives a reference signal from a neighbor basestation; a transmission power calculator that calculates transmissionpower by using the received reference signal; and a power controllerthat transmits the reference signal to the neighbor base station atpredetermined power and transmits a signal to an user equipment in acell based on the transmission power calculated in the transmissionpower calculator.
 13. The base station of claim 12, wherein thereference signal receiver directly receives the reference signal orreceives the reference signal through a receiving apparatus connected tothe base station.
 14. The base station of claim 12, wherein thetransmission power calculator calculates the transmission power byestimating a distance between the base station and the neighbor basestation by using the reference signal, controlling the cell coveragefrom the estimated distance, and obtaining the receiving power having adesired magnitude at a boundary of the cell coverage.